The facilitation of optical protection is an important aspect in any optical network. If protection is not provided, optical signals and the data they carry can be vulnerable to subsystem failure, component failure, or otherwise any breakdown of the physical path traversed by the optical signals. Loss of data and degradation of service are just some of the problems associated with these types of failures. In order to provide some protection in the event of a failure, optical protection in the form of optical protection switching to reroute optical signals around a failed optical physical path may be employed.
As with any part of the optical network, optical protection is an important part of an optical switching system, and more so as WDM (Wavelength Division Multiplexed) systems utilize increasingly larger numbers of optical channels. Due to the multichannel nature of the components within the network, the failure of a single component has the potential to effect a large number of different channels at the same time.
A typical photonic-switched wavelength-grooming network node consists of a set of photonic line cards and a set of photonic switching fabric cards. Photonic switching fabric cards often comprise an array of 2D or 3D MEMS (Micro-Electro Mechanical Systems) optical switches that perform the optical switching of the node. MEMS optical switches are most commonly derived from micro-mirror technology but may be derived from liquid crystal technology, liquid drop technology, bubble technology, or any one of a number of other optical signal redirecting technologies. Due to the possibility of switching element misalignment or switching element failure within a MEMS switching fabric, to protect the system against failure, a mechanism for re-routing optical signals from the normal switching fabric to a redundant switching fabric has been developed.
Co-pending U.S. application Ser. No. 09/726,027 describes such a protection switching system in which a MEMS micro-mirror photonic switch chip provides protection of photonic switch fabric cards by rerouting optical signals to a redundant switch fabric.
Generally, there are at least two distinct types of photonic line cards for use with a photonic node, a trunk type and a tributary type.
A photonic trunk line card has an ingress side, into which light of multiple wavelengths comes from the network fiber, passes through an optical amplifier, is demultiplexed and each wavelength sent to a respective photonic switching fabric card. Variable optical attenuators (VOAs), optical taps, optical monitors and optical supervisory channel (OSC) functions are added as required. Similar and complementary functions (typically including multiplexing) are present on the egress side of each photonic trunk line card, from which the light exits and proceeds into the network fiber. It should be noted that there are many ways in which a photonic trunk line card may be partitioned so that it is capable of servicing one or more network fibers each carrying one or more wavelengths. As is proposed in U.S. application Ser. No. 09/726,027, photonic switches built using MEMS arrays are utilized in the trunk cards to provide a redundant optical path to a redundant switch fabric card, so that in the event that an initially designated switch card of the system fails and needs to be protected, the protection module of the trunk card (via MEMS based switches) provides the redundant optical path to a redundant switch fabric card.
A photonic tributary line card is similar to a photonic trunk line card, except that it handles several network fibers, with each network fiber carrying only one single wavelength channel optical signal, and hence no demultiplexing or multiplexing function is required. For the purposes of this document a single channel optical signal centered on a wavelength λ is denoted by the center wavelength λ. Each optical signal, before it is sent to a photonic switch fabric card, may pass through a protection switch as described, for example, in Co-pending U.S. application Ser. No. 09/726,027, and can be rerouted to a redundant photonic switch card if required.
A switching arrangement generally can include diverse assortments of ingress cards (ingress trunk line cards and ingress tributary cards) on the ingress side of a switching fabric and diverse assortments of egress cards (egress trunk line cards and egress tributary cards) on the egress side of the switching fabric, in which optical signals may be switched from any ingress card to any egress card as required.